LAST EDITED ON Jun-07-20 AT 01:25 PM (EDT)
 
>They couldn't have made Nethroi, Apex of Death (that's the non-Toho
>name of the Biollante card) a Plant instead of a Cat?

The original design for the Mutate mechanic required a significant tribal component to work - you would only have been allowed to Mutate onto a creature that shared either a creature type or one of a handful of keywords. So a flying Cat could Mutate onto another Cat or a flying Squirrel, but not a Brushwagg. The decision was therefore made to pick a different tribe for each colour, and for each of the apex legends to be the types matching their colours. Cats were chosen for white, Nethroi is white, therefore Nethroi is a Cat (Nightmare Beast).

Later on, it was decided this was over-complicating what is already possibly the most complicated mechanic since Banding, so the restriction on what you could Mutate onto was changed to any non-Human creature, and most of the tribal elements were dropped from the set, the apex legends' types being one of the only clues they had ever been there in the first place.